Copending application Ser. No. 760,339, the disclosure of which is incorporated herein by reference and made a part hereof, discloses a phase responsive color video signal correction system which allows correction of the spectral distribution and luminance aspects of a phase-encoded composite video signal. U.S. Pat. No. 4,096,523 to Belmares-Sarabia, the disclosure of which is incorporated herein by reference and made a part hereof, discloses another technique for color correcting video signals wherein the primary color video signals are separated into six independent primary and complementary video signals over which separate control can be exercised in making color corrections. The latter system is sometimes known as a "six vector" or "multi-vector" system, and is also sometimes known as a "matrix" type color corrector.
While both of the above-referenced video signal control systems allow color correction of input video signals, both systems leave room for advancement in selecting a portion of an image represented by an input video signal for correction. Most color correction systems are generally hue oriented, in that the controls over video parameters such as hue, saturation, and luminance are grouped or organized according to hue. For example, in the Belmares-Sarabia system a panel of hue, saturation and luminance controls is provided, there being a separate hue, saturation, and luminance control for each one of the six primary and complementary colors. In the referenced copending phase responsive video signal control system, there is also a separate hue, saturation, and luminance control for each of the color vectors or fans. Both of these systems may be considered a "control per hue" system.
Although there is greater selectivity in the phase responsive system which allows an operator to select a hue for correction with greater particularity than in the prior art, there is often a need to distinguish between regions in a video image which have the same hue but which may differ in luminance. There are many occasions when a given scene may contain several different regions which have similar hues, but only one region requires color correction. Often, these similarly-colored regions will have different luminances. For example, a color correction system operator may decide that a red dress worn by a subject in a given scene requires a color correction. However, assume that the scene also includes a red soft drink cup and a red tablecloth so close in hue to the red dress that selection of the red dress for a color correction results in imposing undersirable color corrections to the soft drink cup and the tablecloth. Yet, the luminance for all these objects may differ appreciably. The inability of prior art color correction systems to differentiate between objects having the same hue but different luminance leads to possible inefficiencies in operation and slows the color correction process, because the operator must repeatedly adjust controls in order to achieve an aesthetically pleasing balance between adjustments for the red dress, soft drink cup, and tablecloth.
Accordingly, there is a need for color video control circuitry which is able to select a portion of a video image for colorimetry correction by allowing discrimination between portions of a video image which have similar hues but which may differ in luminance.